fix(NIP-01): allow limits on a per-filter basis

The original implementation of subscription limit applied to the
entire query, instead of the specific filter.  Now, each filter gets
its own query limit.  When a limit is applied, the most recent N
events will be returned, otherwise the default is to return the
earliest events (in order), for all matching events.

src/subscription.rs

@@ -32,7 +32,7 @@ pub struct ReqFilter {
     /// List of author public keys
     pub authors: Option<Vec<String>>,
     /// Limit number of results
-    pub limit: Option<u32>,
+    pub limit: Option<u64>,
     /// Set of tags
     #[serde(skip)]
     pub tags: Option<HashMap<String, HashSet<String>>>,